Heating Systems and Equipment
Candidates must be able to match heat sources to distribution and to explain combustion, electric-resistance, and hydronic heating in plain language. Know the difference between steady-state efficiency and AFUE: AFUE accounts for cycling, jacket, and off-cycle losses, so a furnace rated 80% AFUE delivers less usable heat than its steady-state combustion efficiency suggests. Be able to identify the major components of a gas-fired forced-air furnace (burner manifold, heat exchanger, inducer, gas valve, limit, flame sensor) and the sequence of operation: call for heat, inducer proves draft via pressure switch, ignition, flame sensed, blower delay, then blower on temperature rise.
Traps in questions include confusing temperature rise with supply-air temperature, mixing up the high limit (safety, opens on overheat) with the fan limit (controls blower on/off), and assuming a condensing furnace can vent in ordinary plastic at any slope. Remember that condensing appliances require Category IV venting with proper slope back to the appliance and a condensate drain with trap.
Hydronic systems: know series-loop vs. primary/secondary piping, the function of an air eliminator and expansion tank, and why circulator sizing depends on head loss, not just flow. Electric heat: strip heaters are 100% efficient at point of use but expensive to operate; sequencers stage elements.
Rule of thumb: For a gas furnace, target a temperature rise near the middle of the nameplate range (commonly 35–65°F). If measured rise is high, suspect low airflow (dirty filter, undersized duct, wrong blower speed); if low, suspect overfiring or excessive airflow. Worked example: nameplate rise 40–70°F; measured supply 130°F and return 70°F gives 60°F rise - acceptable.